1. Field of the Invention
The present invention relates to a transceiver for carrying out transmission and reception of data by using electric fields induced in an electric field propagating medium, and more particularly to a transceiver to be used for data communications using wearable computers that can be worn on human bodies.
2. Description of the Related Art
Due to the progress in reducing size and improving performance of portable terminals, the wearable computers that can be worn on the living bodies are attracting attentions.
Conventionally, in order to realize the data communications between such wearable computers, there has been proposition to carry out data transmission and reception by connecting transceivers to computers and propagating electric fields induced by these transceivers inside the living bodies which are electric field propagating media (see Japanese Patent Application Laid Open No. 2001-352298, for example).
FIG. 1 shows a configuration of a conventional transceiver. A transceiver 5 shown in FIG. 1 is connected to a wearable computer 7 through an I/O circuit 501 for carrying out signal input and output, and has a transmission and reception electrode 511 provided in proximity to a living body 9 through an insulator 513. The data transmitted from the wearable computer 7 modulates alternating current signals generated by an oscillator 507 as carriers in a modulator 505. The modulated signals induce electric fields in the living body 9 from the transmission and reception electrode 511 through the insulator 513, and these electric fields are propagated inside the living body 9, to propagate the data transmitted from the wearable computer 7 to a transceiver 5 provided at the other portion of the living body 9 or a transceiver 5 that is electrically connected by a contact with the living body 9.
When another transceiver 5 receives the electric fields so propagated through the transceiver 5, the electric fields received by the transmission and reception electrode 511 through the insulator 513 are converted into electric signals by an electric field detecting optical unit 515, and the electric signals are supplied to a signal processing circuit 517. The signal processing circuit 517 applies a signal processing such as filtering and amplification to the electric signals from the electric field detecting optical unit 515. After the signal processing, the demodulation and the waveform shaping of the data are carried out by a demodulation circuit 523 and a waveform shaping circuit 525 respectively, and the signals applied with these series of processings are transmitted from the I/O circuit 501 as the received data of the wearable computer 7.
In this way, the transceiver 5 to be used for the data communications between the wearable computers 7 induces the electric fields based on data to be transmitted in the living body 9 which is an electric field propagating medium and carries out the transmission of the data by using these induced electric fields, while receiving signals by using the electric fields induced in the living body 9 at a time of receiving the data.
FIG. 2 shows an exemplary case of using such wearable computers by wearing them on a human body which is an example of the living body 9. As shown in FIG. 2, the wearable computers 7a, 7b and 7c are put on arms, shoulders, torso, etc., of the human body through respectively connected transceivers 5a, 5b and 5c to carry out mutual data transmission and reception. In addition, when tip ends of a hand and a leg are in contact with transceivers 5a′ and 5b′ that are connected to an external terminal 80 through a cable 90, it is possible to carry out data transmission and reception between the wearable computers 7a, 7b and 7c and the external terminal 80.
As shown in FIG. 3, the transmission circuit 503 that is driven without using an AC power source in the above described transceiver 5 is separated from an Earth ground 51, so that a parasitic capacitance 43 appears between a ground for transmission circuit 41 and the Earth ground 51. Also, a parasitic capacitance 53 exists between the living body 9 and the Earth ground 51, and (virtual capacitors having) these two parasitic capacitances appear to be connected in series from a viewpoint of the modulation circuit 505.
For this reason, a voltage Vs between the transmission circuit 503 and the ground for transmission circuit 41 is applied in division over these two parasitic capacitances 43 and 53. Here, by denoting values of the parasitic capacitances 43 and 53 as Cg and Cb respectively, a voltage Vb applied to the living body 9 can be expressed by the following equation (1):
                    Vb        =                              Vs            ·                                          1                                  j                  ⁢                                                                          ⁢                  ω                  ⁢                                                                          ⁢                  Cb                                                                              1                                      j                    ⁢                                                                                  ⁢                    ω                    ⁢                                                                                  ⁢                    Cb                                                  +                                  1                                      j                    ⁢                                                                                  ⁢                    ω                    ⁢                                                                                  ⁢                    Cg                                                                                =                      Vs            ·                          Cg                              Cb                +                Cg                                                                        (        1        )            where j is an imaginary unit (−1)1/2 and ω is an angular frequency of the applied voltage.
In the case of utilizing the AC power source, the parasitic capacitance 43 can be regarded as infinite, so that it becomes Vb=Vs as apparent from the equation (1) and the signals will be applied to the living body 9 without any attenuation. On the other hand, in the case of not utilizing the AC power source, it becomes Vb<Vs according to the equation (1) so that there has been a problem that signals applied to the living body 9 will be attenuated.